Conventionally, physical distribution control systems which collect, and distribute packs have relied on people's hands for a large amount of work. There is known a system which uses bar codes to reduce work by people's hands. However, a small amount of information can only be treated with bar codes and complex automation of the management system cannot be achieved. Moreover, because it is necessary to cause a reader to extremely approach a bar code to read same, work by people's hands will still appear.
Therefore, physical distribution management systems using a tag ID card are proposed on which a large amount of information is recordable and from which information is readable from a position remote from the card in a non-contact manner (JP-A-8-268513).
It is important that such a physical distribution management system reads information accurately from the ID card in a non-contact manner.
U.S. Pat. No. 4,983,976 discloses a moving object identification device which uses electromagnetic waves. In this prior art, electromagnetic waves are radiated from an antenna installed on a question unit toward a response unit at a distance of about 50 cm to 1 m for communicating purposes.
In this case, as the distance between the question and response units increases, the reliability of the communication decreases. While a pack to which a response unit is affixed is moving on a belt conveyer, it is difficult to radiate the electromagnetic waves from the antenna surely to the response unit surely due to the directivity of the antenna, and the reliability of the communication would be reduced.
In addition, when a plurality of response units exist in the coverage of the question unit, it is necessary to identify respective response signals from the plurality of response units. Japanese patent No. 2534295 discloses a technique in which a response unit sends an identification code to a question unit in response to a question signal from the question unit, the question unit sends back to the response unit an affirmative or negative determination signal representing whether the identification of the identification code has succeeded or not, added with an ID number inherent to the response unit, the question unit which has received the affirmative signal does not respond to questions from the question unit thereafter for a predetermined period of time to thereby allow the plurality of response units to be identified sequentially. However, in the prior art, a problem that the question unit cannot normally identify the response units because identification codes from the response units can be sent simultaneously to the question unit and interfere with each other is not considered.
JP-A-9-44614 discloses a technique for preventing response signals from interfering with each other. In this prior art, each question unit sends a one-bit ("0" or "1") signal randomly. A response unit compares the first bit of an ID code inherent to the response unit and the sent random bit. Receiving the following random bit, the response unit compares that random bit and the second bit of the ID code. If the random bit coincides with the first bit of the ID code inherent to the response unit, the response unit compares that random bit with the second bit of the ID code. If the random bit does not coincide with the first bit of the ID code, the response unit again compares that random bit with the first bit of the ID code. This operation is repeated sequentially. When the final bit of the ID code coincides with the random bit, the response unit sends a response signal to the question unit to control the congestion of the response signals. However, the congestion of the response signals cannot be completely removed because there is a possibility that the response signal will be sent to the question unit in response to the same random bit in this method. Especially, as the number of response units used increases, the probability that response signals will be sent simultaneously to the question unit increases. Thus, the reliability of identifying the respective response units would decrease.